The present invention relates to an interferometric analyzer for detecting several different substances in a blend, and more particularly the present invention relates to multiple substance, interferometric analyzers under the assumption that the substances being present in a blend have a particularly structured absorption spectrum such as a periodic or quasiperiodic spectrum, or at least a well defined band edge.
Equipment of the kind to which the invention pertains includes usually a source of radiation having and providing a radiation path in which is positioned a sample of the substance to be investigated and in which particular substances of interest are presumably included; the equipment moreover may include a selective filter and a detector which converts radiation into an electrical signal that is representative of the information sought.
German printed patent application 32 06 472 discloses a measuring device for the optical gas analysis of plural (n) different components in a blend. A particular example elaborated on is related to the detection of carbon monoxide, sulfur dioxide and water. Here one uses the method of comparing wavelengths. A rotating filter wheel with several different optical filters establishes a certain selectivity concerning particular selective wavelengths in the radiation. The time multiplexed output of the detector is processed in a computer facility which calculates the radiation extinction as far as it is contributed to by the various components.
This particular device is disadvantaged by the fact that it has only a limited optical selectivity. All other, that is parasitic and thus interfering absorption through companion gases, and depending on their respective absorption spectrum, are also and more or less indiscriminately ascertained. Thus the interference components have to be compensated in some fashion, for example calculated out of the result in an algebraic process, namely by calculating results using a system of algebraic equations. This calculation will yield exact results only if one assumes the validity and applicability without limitation of the Lambert-Beer absorption law. That is sometimes more or less the case but sometimes it is not. Problems result when the companion gas concentration varies to a considerable degree so that in fact now deviations from the Lambert-Beer law do occur.
The German printed patent application 36 25 490 describes a multicomponent process analyzer system which uses a disperging optic in form of a grating grid monochromatic analyzer for spectrum analysis and separation. Here a particular use is made of an imaging element realized by a slit. Inherently, then this device is of low intensity and marginal yield. Moreover for compensation and eliminating the effect of interfering companion gas in the absorption, it is also here necessary to solve a more or less complex equation system.
Still another device for determining several components in an exhaust gas blend is described in German printed application 25 59 806. The device uses also here a filter wheel for spectral separation of various components with follow-up and supplemental calculation to obtain corrections. The disadvantages outlined above are also applicable because they refer to this kind of system in principle, irrespective of construction details.
Another multicomponent analyzer is known for example through the company ELF AQUITAINE described in a prospectus called "Gas analyzer using Optical Interferometry" published by that company - L Cdex Paris. The device is provided here for the detection of several substances under utilization of a characteristic optical path difference, and the device employed includes the following elements. There is a Wollaston prism used as a polarizer then there is a photoelectric modulator, a birefringent plate element having a thickness that corresponds to the characteristic path difference of the substances; next is provided a second Wollaston prism as an analyzer; and finally, there is a disperging element which is a holographic grid with several associated output slits and detector elements.
It was found that for gas analyses in the infrared region and for simultaneously detecting carbon monoxide, nitrogen monoxide and sulfur dioxide there is no adequate material available that does have the requisite birefringent characteristics, with an adequate degree of transmissivity; so that this device no matter what its value is not applicable for measuring the detection of these particular gas components.